At present, the standardization of Long Term Evolution-Advanced (LTE-A) by Third Generation Partnership Project (3GPP) is ongoing in relation to mobile communication technology. LTE-A is a standard into which LTE has developed.
In LTE, a mobile station communicates with one base station. On the other hand, in LTE-A, technology of coordinated multiple point transmission and reception (CoMP) is used. CoMP is technology in which a plurality of base stations share information and communicate with a mobile station in a coordinated manner.
User throughput or cell throughput at a cell edge is expected to be improved in LTE-A compared to LTE (Non-Patent Document 1). The use of CoMP in both downlink (DL) and uplink (UL) has been studied.
As a method using CoMP in DL, two types of methods have been mainly studied.
Like LTE, in the first method, a mobile station communicates with a base station one-to-one, but a plurality of neighboring base stations share information and perform scheduling or beam-forming in a coordinated manner, thereby reducing interference.
Unlike LTE, in the second method, a plurality of base stations simultaneously transmit signals to one mobile station, and the mobile station combines and demodulates the signals, thereby improving reception quality.
In addition, a method in which a plurality of base stations receive and combine signals transmitted by mobile stations, thereby improving reception quality, has been studied as a method using CoMP in UL.
Here, physical layers of LTE and LTE-A will be described. In the physical layers, physical channels and physical signals are defined. In DL physical channels, six types of a physical DL shared channel (PDSCH), a physical broadcast channel (PBCH), a physical multicast channel (PMCH), a physical control format indicator channel (PCFICH), a physical DL control channel (PDCCH), and a physical hybrid automatic repeat request (HARQ) indicator channel (PHICH) are defined.
In DL physical signals, two types of a reference signal (RS) and a synchronization signal are defined.
In UL physical channels, three types of a physical UL shared channel (PUSCH), a physical UL control channel (PUCCH), and a physical random access channel (PRACH) are defined.
In a UL physical signal, one type of RS is defined.
PDSCH and PUSCH are physical channels mainly for transmitting user data or control data. PBCH is a physical channel for transmitting broadcast information. PMCH is a physical channel for transmitting multicast data such as a broadcast.
PCFICH is a physical channel for notifying of the number of symbols of PDCCH. PDCCH is a physical channel for transmitting a scheduling or transmission power control (TPC) command and the like of PDSCH or PUSCH. PHICH is a physical channel for transmitting ACK/NACK of HARQ for PUSCH.
PUCCH is a physical channel for transmitting ACK/NACK of HARQ for PDSCH, channel quality information (CQI), a precoding matrix indication (PMI), a rank indication (RI), and the like. PRACH is a physical channel for transmitting a preamble of random access.
In addition, UL and DL RSs are physical signals to be used for channel estimation or a CQI measurement. The synchronization signal is a physical signal to be used for a cell search.
In mobile communication, when a mobile station moves during communication (during voice communication, during data communication, or the like), a handover process for changing a base station to communicate with the mobile station is performed. The handover process in LTE will be described with reference to FIGS. 14 and 15 (Non-Patent Document 2).
In state 1 of FIG. 14, signals to be transmitted/received between base stations 100A and 100B and a mobile station 200 before a handover in LTE are shown.
In state 1 of FIG. 14, the base station 100A transmits a signal or channel of each of PDSCH, PCFICH, PDCCH, PHICH, and RS to the mobile station 200 using DL.
In addition, in state 1 of FIG. 14, the mobile station 200 transmits a signal or channel of each of PUSCH, PUCCH, and RS to the base station 100A using UL.
In addition, in state 1 of FIG. 14, the base station 100B transmits RS to the mobile station 200 using DL.
In state 2 of FIG. 14, signals to be transmitted/received between the base stations 100A and 100B and the mobile station 200 after the handover in LTE are shown.
In state 2 of FIG. 14, the base station 100A transmits RS to the mobile station 200 using DL.
In addition, in state 2 of FIG. 14, the base station 100B transmits a signal or channel of each of PDSCH, PCFICH, PDCCH, PHICH, and RS to the mobile station 200 using DL.
In addition, in state 2 of FIG. 14, the mobile station 200 transmits a signal or channel of each of PUSCH, PUCCH, and RS to the base station 100B using UL.
FIG. 15 is a sequence diagram showing processes of the mobile station 200 and the base stations 100A and 100B in LTE.
First, the base station 100A instructs the mobile station 200 to measure reception qualities, reception timings, or the like of signals transmitted from peripheral base stations including the base station 100A (step S3001).
On the basis of the instruction of step S3001, the mobile station 200 measures the reception qualities, the reception timings, or the like of signals transmitted from the peripheral base stations (step S3002). In the reception quality measurement, a reception level, path loss, signal to noise ratio (S/N), or the like of RS is used.
The mobile station 200 transmits measurement results including the reception qualities or the reception timings measured in step S3002 to the base station 100A (step S3003).
The base station 100A determines whether or not to perform a handover process on the basis of a measurement result report of step S3003 (step S3004). For example, the base station 100A determines to perform the handover process if the reception quality of a signal transmitted by the neighboring base station 100B to the mobile station 200 is better than the reception quality of a signal transmitted by the base station 100A to the mobile station 200.
If the handover process is determined to be performed in step S3004, the base station 100A transmits a handover request to the base station 100B (step S3005).
The base station 100B prepares the handover, and transmits a handover response to the base station 100A to notify the base station 100A of handover preparation completion when the preparation is completed (step S3006).
The base station 100A receiving the handover response from the base station 100B transmits a handover instruction to the mobile station 200 (step S3007).
The mobile station 200 releases communication with the base station 100A (step S3008). The mobile station 200 transmits PRACH to the base station 100B so as to acquire synchronization with the base station 100B (step S3009).
The base station 100B performs a random access process with the mobile station 200. In the random access process, the base station 100B calculates a transmission timing change amount of which an indication is sent to the mobile station 200. Specifically, the base station 100B receives a random access request of step S3009, calculates a difference between reception timing when the base station 100B receives the signal transmitted from the mobile station 200 and reception timing expected by the base station 100B, and calculates the transmission timing change amount from the timing difference (step S3010).
The base station 100B transmits a random access response including the calculated transmission timing change amount to the mobile station 200 (step S3011).
On the basis of the random access response received in step S3011, the mobile station 200 calculates transmission timing directed to the base station 100B based on transmission timing directed to the base station 100A and the transmission timing change amount included in the random access response. The mobile station 200 is wirelessly connected to the base station 100B and initiates communication with the base station 100B (step S3012).
The mobile station 200 transmits a handover completion notification to the base station 100B (step S3013).
Next, DL CoMP in which a plurality of base stations simultaneously transmit data to one mobile station as shown in FIG. 16 will be described.
In state 1 of FIG. 16, signals transmitted/received between base stations 101A and 101B and a mobile station 201 before a handover in CoMP are shown.
In state 1 of FIG. 16, the base station 101A transmits a signal or channel of each of PDSCH, PCFICH, PDCCH, PHICH, and RS to the mobile station 201 using DL.
In addition, in state 1 of FIG. 16, the mobile station 201 transmits a signal or channel of each of PUSCH, PUCCH, and RS to the base station 101A using UL.
In addition, in state 1 of FIG. 16, the base station 101B transmits a signal or channel of each of PDSCH and RS to the mobile station 200 using DL. The base station 101B transmits PCFICH and PDCCH to the mobile station 201, if necessary.
In state 2 of FIG. 16, signals transmitted/received between the base stations 101A and 101B and the mobile station 201 after the handover in CoMP are shown.
In state 2 of FIG. 16, the base station 101A transmits a signal or channel of each of PDSCH and RS to the mobile station 201 using DL. In addition, the base station 101A transmits PCFICH and PDCCH to the mobile station 201, if necessary.
In addition, in state 2 of FIG. 16, the base station 101B transmits a signal or channel of each of PDSCH, PCFICH, PDCCH, PHICH, and RS to the mobile station 201 using DL.
In addition, in state 2 of FIG. 16, the mobile station 201 transmits a signal or channel of each of PUSCH, PUCCH, and RS to the base station 101B using UL.
In state 1 of FIG. 16, the base station 101A receives UL data from the mobile station 201. When the mobile station 201 moves, a handover process is performed, for example, if UL quality directed to the base station 101B is better than UL quality directed to the base station 101A. In the handover process, a base station, which receives UL data transmitted from the mobile station 201 and transmits DL control channels (PCFICH, PDCCH, and PHICH) to the mobile station 201, is changed from the base station 101A to the base station 101B.
On the other hand, DL data is not changed before/after the handover process. That is, the mobile station 201 receives the DL data from both the base stations 101A and 101B. In this state, the mobile station 201 needs to release communication so as to perform the random access process when the handover process is performed in LTE. At this time, the mobile station 201 should stop communication even in DL data that does not need to be changed.